Re: Which 6502 variant to use?

[mjmahon@aol.com (Michael J. Mahon)]

Dave Althoff Jr wrote:

<snip>

>The Monitor ///, on the other hand, was an abomination from the day it was
>first connected to an Apple ][.  It's built to sit on top of a proFile
>which is built to sit on top of an Apple /// (which was so successful that
>I am surprised that Apple ever introduced any computer with a "3" in its
>name ever again.  How many years passed between the Apple /// and the
>Macintosh LC-III?).  It won't sit nicely on a ][ unless you have a stand,
>it is dark, its screen is impossible to clean with the glare shield in
>place, and if you remove the glare shield with a razor blade, it is
>impossible to read.  

Although the Monitor III has its drawbacks, it is a quite readable
monitor if it has sufficient brightness.

I have encountered a Monitor III with very low brightness, although
its display was normal in every other respect.  As a TV repair guy
from the 1950s, I recognized this as a case of low emission from
the CRT cathode.

In the early days, this problem was not uncommon, being caused
by poor ion traps leading to heavy ion bombardment of the cathode
surface.  The actual emitting surface of a thermionic cathode is only
a few microns deep, so any surface change has a huge effect.

The usual remedy was either a temporary boost in filament voltage,
to "burn off" the cathode surface and create a more emissive surface,
or a smaller, permanent boost in heater voltage, to increase the
temperature of the surface and improve electron emission.

In the case of my Monitor III, I took the latter course, since the
heater is powered by 12v, and there is an unregulated 15v supply
available.  I simply rewired the heater to the 15v supply, and the
available brightness was sufficient that the "brightness" control
could be turned to the normal part of its range.

If anyone else has a similar problem with a Monitor III, they
might consider making this change.

BTW, increasing heater voltage over "rating" will reduce its
lifetime, but, in my experience, by the 1970s, CRT heaters
were very durable, to ensure that heater burnout was not the
cause of an expensive CRT failure.

-michael

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